1. Field of the Invention
The present invention relates to a method of measuring a radius of curvature of a probe of a cantilever used in a scanning probe microscope.
2. Description of the Related Art
In a scanning probe microscope, the tip shape of a probe directly involves a contact state with a sample, and hence it is essential to measure the tip shape of the probe in advance in order to grasp the measurement performance in shape measurement or physical properties measurement of a sample surface. In many cases, in fact, with the assumption that the probe sip is semi-spherical, the sharpness of the probe tip is represented by the radius or the diameter of the hemisphere rather than the tip shape of the probe itself.
Conventionally-used means for measuring the tip shape of the probe are observation by an electron microscope and a method of measuring the shape of a sharp needle-like sample (hereinafter referred to as “probe shape test sample”)by a scanning probe microscope to draw the tip shape of the probe. The latter method of using a probe shape test sample 3b has an advantage of being simple and not needing another device because the tip shape can be measured under the state where a cantilever is mounted on the scanning probe microscope. This method is therefore generally and widely used.
For example, as described in Japanese Patent Application Laid-open No. 2001-165844, there is known a method in which, based on the shape of a probe 2a treasured by the probe shape test sample Sb, the radius of the hemisphere (typically called radius of curvature) of the probe tip is calculated from the angle of a side surface of the probe 2a and the width of a curved portion of the tip of the probe 2a. Alternatively, as described in JIS R1683, “Test method for surface roughness of ceramic thin films by atomic force microscopy”, there is known a method in which the tip shape of the probe 2a is measured by the probe shape test sample Sb by the scanning probe microscope in such a manner that the tip shape of the probe is measured by a needle of the probe shape test sample at a sharp needle-like protruding tip of the probe shape test sample Sb as illustrated in FIG. 6, and then the diameter of a cross-section at a given distance from the apex of the tip shape of the probe 2a is determined (FIG. 8), to thereby evaluate the tip shape of the probe 2a. 
In the probe shape evaluation method described in Japanese Patent Application Laid-open No. 2001-165844, the side surface of the probe 2a needs to be linear. The actual probe 2a is, however, a microstructure, and hence the side surface is not always manufactured to have a linear shape. Therefore, the boundary between the side surface and the curved portion of the tip cannot be determined accurately, and it is difficult to determine the radius of curvature of the tip by this method.
In the probe shape evaluation method described in JIS R1683, “Test method for surface roughness of ceramic thin films by atomic force microscopy”, the evaluation is performed in such a manner that, on the premise that the probe tip has a cylinder shape in which the width is substantially identical even on the root side of the hemisphere of the tip, the diameter of such cylinder part is regarded as the diameter of the hemisphere of the tip. However, the required preconditions in this evaluation method are that the height at which the shape is measurable by the above-mentioned probe shape test sample Sb is larger than the radius of curvature and that the root part of the hemisphere has a shape that can be approximated as a cylinder having a constant diameter. However, there are various kinds of probe shapes depending on their uses. In some of the probes, the hemisphere of the tip is enlarged by design, while in others, the root part of the hemisphere of the probe has a tapered shape rather than a cylinder. The method described in JIS R1683, “Test method for surface roughness of ceramic thin films by atomic force microscopy” may be inapplicable to the probe having such a special shape. For example, as illustrated in FIG. 7, in the case where the radius of curvature of the probe is larger than the height of a convex portion of the probe shape test sample Sb, that is, larger than the height at which the shape of the probe is measurable, the probe shape in a region At for measuring the probe shape is traced by the probe shape test sample only for a part of the hemisphere of the tip as illustrated in FIG. 9. Further, in this case, even when the diameter of the cross-section at a predetermined distance from the apex of the probe tip shape is determined as described in JIS R1683, “Test method for surface roughness of ceramic thin films by atomic force microscopy”, the determined diameter is not coincident with the diameter of the hemisphere of the probe tip.